Natural gas prices have declined over the past week due to record-high storage levels and mild winter weather forecasts.

That decline in gas narrowed the relative price difference between NYMEX Central Appalachian coal and NYMEX Henry Hub natural gas to below $1 per million British thermal units (mmBtu) for the first time since September, according to Reuters data.

Natural gas traded at $3.18/mmBtu Thursday morning, while Eastern coal was selling for about $2.20/mmBtu.

Energy traders said it costs about $1/mmBtu to transport Eastern coal. So when natural gas prices are less than $1 over coal, traders said it starts to make economic sense for generators to burn gas rather than coal.